Generally, fine pattern formation is carried out by the photolithography in manufacturing processes of a semiconductor device. A number of substrates called photomasks (hereinafter referred to as transfer masks) are normally used for this fine pattern formation. The transfer mask comprises generally a transparent glass substrate having thereon a fine pattern made of a metal thin film or the like. The photolithography is used also in the manufacture of the transfer mask.
In the manufacture of a transfer mask by the photolithography, use is made of a mask blank having a thin film (e.g. a light-shielding film or the like) for forming a transfer pattern (mask pattern) on a transparent substrate such as a glass substrate. The manufacture of the transfer mask using the mask blank comprises an exposure process of applying required pattern writing to a resist film formed on the mask blank, a developing process of developing the resist film according to the required pattern writing to form a resist pattern, an etching process of etching the thin film according to the resist pattern, and a process of stripping and removing the remaining resist pattern. In the developing process, a developer is supplied after applying the required pattern writing to the resist film formed on the mask blank to dissolve a portion of the resist film soluble in the developer, thereby forming the resist pattern. In the etching process, using this resist pattern as a mask, an exposed portion of the thin film, where the resist pattern is not formed, is dissolved by dry etching or wet etching, thereby forming a required mask pattern on the transparent substrate. In this manner, the transfer mask is produced.
For miniaturization of a pattern of a semiconductor device, it is necessary to shorten the wavelength of an exposure light source for use in the photolithography in addition to the miniaturization of the mask pattern formed in the transfer mask. In recent years, the wavelength of an exposure light source in the manufacture of a semiconductor device has been shortened from KrF excimer laser (wavelength 248 nm) to ArF excimer laser (wavelength 193 nm) and further to EUV (wavelength 13.4 nm).
As a type of transfer mask, a halftone phase shift mask is known apart from a conventional binary mask having a light-shielding film pattern made of a chromium-based material or the like on a transparent substrate. This halftone phase shift mask is configured to have a phase shift film on a transparent substrate. This phase shift film is made of, for example, a material containing a molybdenum silicide compound or the like and is adapted to transmit light having an intensity that does not substantially contribute to exposure (e.g. 1% to 30% at an exposure wavelength) and to provide a predetermined phase difference. By means of light-semitransmissive portions formed by patterning the phase shift film and light-transmissive portions formed with no phase shift film and adapted to transmit light having an intensity that substantially contributes to exposure, the halftone phase shift mask provides a relationship in which the phase of the light transmitted through the light-semitransmissive portions is substantially inverted with respect to the phase of the light transmitted through the light-transmissive portions. As a consequence, the lights having passed near the boundaries between the light-semitransmissive portions and the light-transmissive portions and bent into the others' regions due to the diffraction phenomenon cancel each other out. This makes the light intensity at the boundaries approximately zero to thereby improve the contrast, i.e. the resolution, at the boundaries.
In recent years, there have also appeared a binary mask blank for ArF excimer laser using a material containing a molybdenum silicide compound as a light-shielding film, and so on.
The miniaturization of the mask pattern formed in the transfer mask requires a reduction in the thickness of the resist film in the mask blank and dry etching as a patterning technique in the manufacture of the transfer mask.
However, the reduction in the thickness of the resist film and the dry etching have the following technical problems.
One problem is that the processing time of, for example, the light-shielding film exists as one major restriction to the reduction in the thickness of the resist film of the mask blank. When chromium, for example, is used as a material of the light-shielding film, a mixed gas of chlorine gas and oxygen gas is used as an etching gas in dry etching of chromium. When patterning the light-shielding film by dry etching using the resist pattern as a mask, since the resist film is an organic film composed mainly of carbon, it is very weak against an oxygen plasma forming a dry etching environment. While patterning the light-shielding film by dry etching, the resist pattern formed on the light-shielding film should remain with a sufficient thickness. As one index, in order to make excellent the cross-sectional shape of the mask pattern, the resist film is required to have a thickness that still remains even when the etching time is about twice a just etching time (100% overetching). For example, since, in general, the etching selectivity of chromium as the material of the light-shielding film to the resist film is 1 or less, the thickness of the resist film is required to be twice or more that of the light-shielding film. Therefore, it is necessary to shorten the processing time of the light-shielding film for reducing the thickness of the resist film and, for that purpose, it is important to reduce the thickness of the light-shielding film. However, while reducing the thickness of the light-shielding film, the light-shielding film is required to have a predetermined optical density (normally 3.0 or more at a wavelength of exposure light for use with a mask) for ensuring its light-shielding performance and therefore the reduction in the thickness of the light-shielding film has its own limitation.
In view of this, as a method of reducing the thickness of the resist film, there has conventionally been proposed a method of forming, on a light-shielding film, an etching mask film made of a material having an etching selectivity to the light-shielding film, then etching the etching mask film using a resist pattern as a mask to form an etching mask film pattern, and then etching the light-shielding film using the etching mask film pattern as a mask to form a light-shielding film pattern (e.g. Patent Document 1 etc.).